|Fig. 1: Nuclear Power Plant. (Source: Wikimedia Commons.)|
Nuclear power is a significant source of electricity, with just 435 nuclear plants supplying 16% of the world's power in 2005. In the United States, where 29.2% of the world's reactors reside, nuclear facilities account for 19% of the national electricity generation. In France, 79% of electricity is obtained from nuclear sources, and nuclear energy contributes to over 20% of the national power production in Germany, Japan, South Korea, Sweden, Ukraine, and the United Kingdom. 
Despite the fact that the heat and electricity generating cycles do not cause greenhouse gas emissions, nuclear energy is not a zero emission energy source. The extensive system of upstream supply stages requires energy input, and since a major part of energy inputs is by fossil fueled sources, nuclear energy involves the emission of greenhouse gases. 
The nuclear-fuel cycle is composed of mining and milling uranium ores, fuel conversion, enrichment, fabrication, and nuclear power plant construction, operation, reprocessing, as well as waste disposal. Once the uranium is mined at the surface or underground, the ore is crushed and leached in sulfuric acid. 
Life cycle GHG emissions are wide-ranged in estimates in recent studies, ranging from 3.5 to 100 grams of CO2 equivalent per kWh. Such wide range is due to differences in enrichment, production, and operation. The processes of diffusion versus centrifugation, as well as country-specific background electricity mixtures result in large differences in GHG emissions during the enrichment phase. For example, the electricity generation in Sweden and Switzerland is mostly fossil-fuel free, with 37% being nuclear. 
The main factors that affect the amount of GHG emissions from a fossil fuel power plant is the type of technology used, the choice of fuel, and the thermal efficiency. Thermal efficiency increases directly with the load factor, meaning that GHG emissions depend on the mode of operation. While thermal efficiency and plant technology influence direct GHG emissions, the carbon content of the fuel also has a great influence. Weisser et al. show how it can stated that typically "the higher the heating value the lower the carbon content of the fossil fuel." 
There is an average of 1-5 g of uranium and 3-20 g of thorium in one tonne of rock and soil. Among the naturally occurring fissile isotopes, only uranium is mined for use in obtaining nuclear fuel. Uranium is extracted from the following methods: ores using open pit (30%), under-ground excavation (38%), in situ leaching (21%), or as a product in other mining (11%). Open pit excavation requires the largest amount of materials to be removed, while it situ requires the smallest amount. The energy intensity per unit of metal product and the recoverable portion of uranium is dependent on the concentration of the metal in the ore. 
Nian et al. introduce a "bottom-up" approach to minimize the margin of uncertainty in calculating the carbon emission factor in nuclear power generation.  Data from the literature shows a common value of carbon emission factor in nuclear power generation differing by more than a factor of 100. The methodology displayed in this paper enables a comparison of carbon emissions from nuclear power generation, and employs the principle of energy balance on a defined power generation system. By using nuclear power as an example, a carbon emission factor of 22.8 grams of CO2 per kWh is obtained, which falls to within 2.5% of the median of globally reported LCA results. 
© Danielle Rasooly. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 B. K. Sovacool, "Valuing the Greenhouse Gas Emissions From Nuclear Power: A Critical Survey," Energy Policy 36, 2940 (2008).
 M. Lenzen, "Life Cycle Energy and Greenhouse Gas Emissions of Nuclear Energy: A Review," Energy Convers. Manage. 49, 2178 (2008).
 V. M. Fthenakis,"Greenhouse-Gas Emissions From Solar Electric- and Nuclear Power: A Life-Cycle Study,"Energy Policy 35, 2549 (2007).
 D. Weisser, "A Guide to Life-Cycle Greenhouse Gas (GHG) Emissions From Electric Supply Technologies," Energy 32, 1543 (2007).
 V. Nian et al., "Life Cycle Analysis on Carbon Emissions From Power Generation - The Nuclear Energy Example," Appl. Energy 118, 68 (2014).